Hydraulic properties of subglacial sediment determined from the mechanical response of water-filled boreholes

Abstract

AbstractFreezing of water-filled boreholes drives water into the subglacial bed and the associated pressure effects yield information about subglacial hydraulic properties. A numerical model describing the mechanical response of an unconnected borehole and the bed beneath it to this freezing forcing was developed, using a nonlinear transient visco-elastic ice-flow law and an approximate model of top-down freezing. The resulting system of equations was solved using the method of lines. Results agreed well with analytic solutions, when parameters were correctly chosen. Forward modelling of pressure records from three 1992 boreholes and three from other years indicated that the till underlying Trapridge Glacier has a hydraulic conductivity of 1.35-7.0 × 10−9m s−1. The model was also used to investigate the response of a borehole to sudden pressure changes. The response is very fast compared to pressure-sensor sampling rates; thus, the true basal signal is essentially unaffected by the presence of the borehole, except during the initial freeze-in.